Calculations
| Appendix B-3
• If ∆A=0, the appropriate amount of air is entering the process.
• If ∆A< 0, there is insufficient air
• If ∆A> 0 there is excess air.
The plant factor in the equation is determined by the concentration of H
2
S
upstream of the Claus Process and the ratio of flows before and after the
Claus Process, i.e
Plant Factor =
[H
2
S]
acid gas
Q
acid gas
Q
t
Air demand is defined as -∆A.
The equation implemented by the Model 888 is shown below:
Air Demand
%
= d {plant factor(Process ratio [SO
2
] - [H
2
S] ) - Excess Air}
The configuration parameters can be found in Chapter 4 in the Config-
ure _Calibration_Special Values menu. These parameters are labeled as
follows:
Delta: Set to ±1
If the end user control method is direct acting, this should
be set to -1 (i.e. air ow should be increased if air demand
> 0).
If the control method is reverse acting set to +1 (i.e. air
ow should be decreased if air demand > 0).
Plant Factor The default value is 3.5.
Process Ratio The desired ratio of H
2
S to SO
2
. The default value is 2.00.
Excess Air
Specied excess air for the process in percent (%). The
default value is 0.0.
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